Solid ink or phase change ink printers conventionally receive ink in a solid form, sometimes referred to as solid ink sticks. The solid ink sticks are typically inserted through an insertion opening of an ink loader for the printer, and are moved by a feed mechanism and/or gravity toward a heater plate. The heater plate melts the solid ink impinging on the plate into a liquid that is delivered to a printhead assembly for jetting onto a recording medium. The recording medium is typically paper or a liquid layer supported by an intermediate imaging member, such as a metal drum or belt.
A printhead assembly of a phase change ink printer typically includes one or more printheads each having a plurality of ink jets from which drops of melted solid ink are ejected towards the recording medium. The ink jets of a printhead receive the melted ink from an ink supply chamber, or manifold, in the printhead which, in turn, receives ink from a source, such as a melted ink reservoir or an ink cartridge. Each ink jet includes a channel having one end connected to the ink supply manifold. The other end of the ink channel has an orifice, or nozzle, for ejecting drops of ink. The nozzles of the ink jets may be formed in an aperture, or nozzle plate that has openings corresponding to the nozzles of the ink jets. During operation, drop ejecting signals activate actuators in the ink jets to expel drops of fluid from the ink jet nozzles onto the recording medium. By selectively activating the actuators of the ink jets to eject drops as the recording medium and/or printhead assembly are moved relative to each other, the deposited drops can be precisely patterned to form particular text and graphic images on the recording medium.
One difficulty faced by fluid ink jet systems is partially or completely blocked ink jets. Partially or completely blocked ink jets may be caused by any of a number of factors including contamination from dust or paper fibers, dried ink, etc. In addition, when the solid ink printer is turned off, the ink that remains in the print head can freeze. When the printer is turned back on and warms up, the ink thaws in the print head. Air that was once in solution in the ink can come out of solution to form air bubbles or air pockets that can become lodged in the ink pathways of the print head. Partially or completely blocked ink jets can lead to ink jet malfunctions or failures resulting in missing, undersized or misdirected drops on the recording media that degrade the print quality.
Some partially or completely blocked ink jets may be recovered by performing a printhead maintenance action. Print head maintenance generally includes purging ink through the ink pathways and nozzles of a print head assembly in order to clear contaminants, air bubbles, dried ink, etc. from the print head assembly and/or wiping the nozzle plate of the print head assembly. To prevent ink and debris from being drawn or pushed back into the printhead via the ink jets during wiping, a low pressure assist (“LPA”) may be applied to the printhead during wiping. The assist pressure is applied to the printhead reservoir during wiping to prevent ink and debris from being sucked into the apertures. To maintain the ink at the apertures, the assist pressure must be sufficient to overcome any back pressure. If the assist pressure is too low ink and debris may be drawn into the apertures. If the assist pressure is too high the apertures may drool ink even after a wipe has been performed. Maintaining the ink at the apertures during a wiping procedure is made more difficult by a varying pressure introduced into the reservoir due to changes in the amount of ink in the reservoir, also referred to as head height. The assist pressure and the head height both serve to apply pressure to the apertures during the wipe. However, the pressure from the ink height is not constant since it changes as the ink level changes.
Surface wetting characteristics and/or internal pressure characteristics of the printhead cause ink to drool from the apertures at a some measurable LPA pressure. If the drool pressure of a printhead is lowered for whatever reason, the application of the normal assist pressure to the on-board reservoir may cause ink to drool from the apertures during wiping. Such drooling can negatively impact imaging operations by leaving ink on the aperture plate which may cause color mixing as well as further contamination of the apertures.